Flaktürme, or Flak Tower, were a series of flak towers that were constructed across Germany during World War II. They were made almost entirely of concrete and rebar and the basic walls of each Flaktürme were about 3.5 meters thick. The towers were constructed in target cities of Allied air raids such as Berlin, Hamburg, and Vienna. More towers were planned in cities such as Munich but were never built. The design of each tower varied between the two types, G and L and also on when they were built. Some were designed personally by Adolf Hitler. Others were designed as improvements on previous designs.